1. Field of the invention
The present invention relates to an amplifier and an optical receiving circuit using the amplifier, and more particularly, to an amplifier for amplifying a burst signal (a signal which intermittently appears) and an optical receiving circuit using the amplifier.
2. Description of the Background Art
In a signal amplifier in an optical receiving circuit handling a continuous signal, a system constructed by AC coupling for detecting the average value of the signal and giving the average value as the level of the threshold value of a comparator for judging one or zero has been employed.
In an optical receiving circuit handling small optical receiving power, signals having values to several millivolts must be amplified, whereby a circuit design capable of restraining an output variation due to a temperature variation and a power supply voltage variation to the utmost is indispensable.
The AC coupling system has been most commonly used because it is easily feasible to stably operate each amplifier because the bias in the amplifier is relatively stable against a power supply voltage variation and a temperature variation.
In recent years, in the field of an optical subscriber system and an optical interconnection system, however, the necessity of handling a burst signal for intermittently exchanging a light signal has been increased, and an optical receiving circuit corresponding thereto has been requested.
Particularly in the optical subscriber system, it has been necessary to consider its application range to include the utilization of outdoors applications, for example, whereby the necessity of ensuring stable operations in a temperature range wider than that in the conventional example has been increased.
When the optical receiving circuit with the conventional AC coupling system receives a burst signal, the average value of a received signal greatly varies at the lead of the burst signal, whereby the signal cannot be accurately reproduced in the vicinity of the lead of the burst signal.
As a receiver handing a burst signal, therefore, a DC coupling system is required. However, the DC coupling system has the disadvantage in that the bias is easily affected by a temperature variation and a power supply voltage variation. This will be described in more detail below.
Conventional examples of an optical receiving circuit using the DC coupling system as a basis include an optical receiving circuit disclosed in Japanese Patent Laid-Open No. 310967/1994 (hereinafter referred to as first prior art), and an optical receiving circuit disclosed in Japanese Patent Publication No. 107943/1995 (hereinafter referred to as second prior art), and optical receiving circuit disclosed in U.S. Pat. No. 5,430,766 (hereinafter referred to as third prior art).
The optical receiving circuit in the first prior art makes it possible to reproduce a pulse signal by using an output signal of a preamplifier to which a photodiode is connected as a signal input of a comparator in the succeeding stage and using an intermediate value produced by resistance-separating an output signal of a first peak detecting unit for detecting and holding the maximum value of an output of the preamplifier and an output signal of a second peak detecting unit for detecting and holding the minimum value of the output of the preamplifier as a reference input of the comparator in the succeeding stage.
The optical receiving circuit in the second prior art makes it possible to reproduce a pulse signal by using an output signal of a first transimpedance preamplifier to which a photodiode is connected as a signal input of a comparator in the succeeding stage and using an intermediate value produced by resistance-separating an output signal of a peak detecting unit for detecting and holding the minimum value of an output of a first preamplifier and an output signal of a second preamplifier (a preamplifier to which no photodiode is connected) for outputting the maximum value of the output of the first preamplifier as a reference input of the comparator in the succeeding stage.
The optical receiving circuits in the first prior art and the second prior art are difficult to utilize in a wide temperature range because a temperature variation in the peak detecting unit directly affects an input of the comparator in the succeeding stage. When noise is included in a signal line, an erroneous peak value may be held in response to the noise in the peak detecting unit. Also in this case, accurate signal reproduction becomes difficult to achieve.
On the other hand, the optical receiving circuit in the third prior art comprises a preamplifier using a differential amplifier and a comparator for obtaining a digital output upon input of a differential output from the differential amplifier. In the optical receiving circuit in the third prior art, an output of a peak detecting unit on the side of a positive terminal of a differential output signal is utilized upon being fed back in order to determine the level of a reference input of the differential amplifier constituting the preamplifier. When there are a temperature variation and a power supply voltage variation, therefore, a variation in the output of the peak detecting unit affects an output of the preamplifier, whereby accurate signal reproduction becomes difficult in a wide temperature range. When noise is included in a signal line, an erroneous peak value may be held in response to the nose in the peak detecting unit. Also in this case, accurate signal reproduction becomes difficult to achieve.
In an optical communication system of passive double star construction or the like in which a plurality of terminals are connected to a star coupler, even when the terminals are temporally separated to make signal transmission utilizing a TDMA technique, the light emission levels at the time of no signal transmission at each terminal , in some cases, are added to become a level which cannot be ignored with respect to a signal level. Further, when a system, in which different signals are transmitted in a multi-wavelength utilizing a WDM technique, is employed, it is considered that a light signal having an unnecessary wavelength is received by a photodetector as background light if the isolation of wavelength division multiplexing light is insufficient in an optical module.
In the optical receiving circuit in the third prior art, two peak detecting units for detecting and holding the respective maximum values of differential outputs of the preamplifier are provided in order to avoid the above-mentioned effect of the background light, to carry out such feedback control as to find the difference between outputs of the two peak detecting units and control a current source so that there is no difference. In such construction, however, two feedback loops are used, whereby the most suitable setting of the time constant involves a difficulty.